The goal of the research is to determine the structure of eukaryotic (rat liver) ribosomes and to understand how the structure accounts for their function in protein synthesis. A good deal is now known about the chemistry of the constituents which has allowed attention to be focused on the second order problem, the interaction of the nucleic acids and proteins. The specific aim of this research is to obtain detailed information on these interactions and to define the molecular architecture of functional domains. For that purpose, use will be made of Alpha-sarcin, a cytotoxic nuclease. Alpha-Sarcin kills cells by inhibiting protein synthesis, the result of the inactivation of ribosomes. The basis for this inactivation is the cleavage of a single phosphodiester bond near the 3' end of 28S rRNA. The plan is to (1) characterize the binding of Alpha-sarcin to ribosomes; (2) to identify the ribosomal proteins in the Alpha-sarcin domain by cross-linking the toxin to them using a bifunctional reagent; (3) to identify the proteins responsible for the immunity of most of the rRNA to attack by Alpha-sarcin; (4) to determine the topographical location of the Alpha-sarcin domain by immune electron microscopy; and (5) to investigate the function of the Alpha-sarcin domain.